+1 on the annealing recommendation. If you still get cracks, try annealing "more" i.e. to a higher temperature. If the metal is glowing a visible dull red in low light, that is about the right temperature. This may be hard to observe when using a torch.
The use of water is to stop the process or limit it. You want the case head to stay hard, and the neck/shoulder to be soft. Some people stand the cases up in a tray of water and run a torch over them. This keeps the heads cool. Dropping a case in water keeps the heat in the neck from transferring to the head. In most situations water is not needed, because the head has enough mass that it will not get hot enough to anneal. The WSSM is one example of where annealing can be tricky, particularly with a torch, because the case is short and thick. I use induction to quickly apply enough heat do the job without heating the entire case.
Cracks are usually due to work hardening or defects/grain structure of the manufacturing process. Stress corrosion cracking, hydrogen embrittlement, neutron embrittlement, mercury inclusion, etc. are not generally a problem for hand loaders.
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